Renewable energy sources include solar energy, wind power, tidal wave energy and the like. A solar power conversion system may include a plurality of solar panels connected in series or in parallel. The output of the solar panels may generate a variable dc voltage depending on a variety of factors such as time of day, location and sun tracking ability. In order to regulate the output of the solar panels, the output of the solar panels may be coupled to a dc/dc converter so as to achieve a regulated output voltage at the output of the dc/dc converter. In addition, the solar panels may be connected with a backup battery system through a battery charge control apparatus. During the day, the backup battery is charged through the output of the solar panels. When the power utility fails or the solar panels are an off-grid power system, the backup battery provides electricity to the loads coupled to the solar panels.
Since the majority of applications may be designed to run on 120 volts ac power, a solar inverter is employed to convert the variable dc output of the photovoltaic modules to a 120 volts ac power source. A plurality of multilevel inverter topologies may be employed to achieve high power as well as high efficiency conversion from solar energy to utility electricity. In particular, a high power ac output can be achieved by using a series of power semiconductor switches to convert a plurality of low voltage dc sources to a high power ac output by synthesizing a staircase voltage waveform.
In accordance with the topology difference, multilevel inverters may be divided into three categories, namely diode clamped multilevel inverters, flying capacitor multilevel inverters and cascaded H-bridge multilevel inverters. Furthermore, multilevel inverters may employ different pulse width modulation (PWM) techniques such as sinusoidal PWM (SPWM), selective harmonic elimination PWM, space vector modulation and the like. Multilevel inverters are a common power topology for high and medium power applications such as utility interface for renewable power sources, flexible ac transmission systems, medium voltage motor drive systems and the like.
The diode clamped multilevel inverter is commonly referred to as a three-level neutral point clamped (NPC) inverter. A three-level NPC inverter requires two series connected capacitors coupled between the input dc buses. Each capacitor is charged to an equal potential. Furthermore, the three-level NPC inverter may comprise four switching elements and two clamping diodes. The clamping diodes help to reduce the voltage stress on the switching element to one capacitor voltage level.
An NPC inverter utilizes a staircase waveform to generate an ac output. Such a staircase waveform resembles a desired sinusoidal waveform. As a result, the output voltage of the NPC inverter may be of a low total harmonic distortion (THD). In addition, the staircase waveform may reduce the voltage stresses. As a result, the electromagnetic compatibility (EMC) performance of the NPC inverter may be improved. In addition, to achieve the same THD, the NPC inverter may operate at a lower switching frequency. Such a lower switching helps to reduce switching losses so as to achieve an efficient power conversion system.